1. Field of the Invention
The present invention relates to the display technical field, and in particular to a backlight driving circuit and a method for reducing the soft-start time thereof.
2. The Related Arts
Liquid crystal display (LCD) has a thin body, energy saving, no radiation and many other advantages to achieve the wide application. Most of the liquid crystal displays in the existing market are backlight-type liquid crystal display, which comprise liquid crystal panel and backlight module. The working principle of the liquid crystal panel is to place liquid crystal molecules in the two parallel glass substrates, there are plurality of vertical and horizontal fine wires between the two glass substrates, to control the rotation of the liquid crystal molecules through applying the driving voltage to the two parallel glass substrates, to generate the screen by refracting the light of the backlight module. Since the LCD panel does not light by itself, it requires borrowing a light source provided by the backlight module to display properly. Therefore, backlight module becomes one of the key components of the LCD device. The backlight module is divided to edge type backlight module and direct type backlight module according to the different location of the incident light. The direct type backlight module is to set the light source such as cold cathode fluorescent lamp (CCFL) or light emitting diode (LED) to the back of the liquid crystal panel, and directly forming the surface light source which is provided to the liquid crystal panel. The edge type backlight module is to set the backlight source LED light bar to the back edge behind the liquid crystal panel, the light emitted from the LED light bar enters into the light guide plate (LGP) from the surface of the incident light on the one side of the LGP, and emits from the light exit surface of the LGP after reflected and diffused, forming the surface light source which is provided to the liquid crystal panel through the optical film group.
Referring to FIG. 1, which is the LED backlight driving circuit according to the prior art, wherein, the constant current driving chip 100 has an internal soft-start function. When the driving power source 200 inputs the driving voltage to the circuit, the duty of the driving signal output by the constant current driving chip 100 to drive the thin film transistor Q100 will slowly open. At the same time, it charges the capacitor C200 on the soft-start, SS, pin 3. When the voltage on the third pin of the SS pin3 reaches a certain value, the constant current driving chip 100 starts to work properly. The function of the soft-start is mainly to prevent that the feedback output voltage of the constant current driving chip 100 is too low when the circuit just boots, which makes the driving signal duty of the thin film transistor Q100 too high, resulting the current through the inductor L100 and the thin film transistor Q100 of the circuit being too high, and damaging the components.
When the constant current driving chip 100 soft-starts, the driving frequency of the driving signal output to the thin film transistor Q100 and that in normal operation are the same. In order to prevent that the switching loss on the thin film transistor Q100 is too high and affects its normal operating temperature, the driving frequency is set lower. Therefore, each cycle time is longer, resulting the soft-start time being longer, and affecting the boot speed of the display device, such as television, etc.